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NERVOUS SYSTEM Chapter 48-49
Nervous System
Function: coordinates and controls bodily functions
with nerves and electrical impulses
The system is composed of different types of nerve
cells called neurons
One neuron may communicate with thousands of other
neurons
Communication between neurons can be long-distance
electrical signals or short-distance chemical signals
Nervous System
In all vertebrates, the nervous
system shows a high degree of
cephalization and distinct CNS
and PNS components
The brain provides the integrative power that
underlies the complex behavior of vertebrates
The spinal cord integrates simple responses to
certain kinds of stimuli and conveys
information to and from the brain
Figure 48.19
Central nervous
system (CNS) Peripheral nervous
system (PNS)
Brain
Spinal cord
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
Information Processing
The nervous system processes information through
detection, generation, transmission, and integration of
signal information
Essentially: Sensory input, integration, and motor output
Figure 48.3
Sensor
Effector
Motor output
Integration
Sensory input
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
Divisions of the Nervous System
2 main divisions are the Central and Peripheral
Nervous systems – CNS and PNS
The CNS integrates and processes information from
the body
The PNS transmits information to and from the CNS
Peripheral Nervous System
Divisions of PNS:
Sensory and Motor division
Sensory = sends signals to the CNS from receptors
Motor = send signals away from the CNS to the parts of the
body
Motor division can be separated into the Somatic nervous
system and the Autonomic nervous system – SNS and ANS
Autonomic nervous system divides into Parasympathetic and
Sympathetic divisions
Peripheral Nervous System
Somatic nervous system
Carries signals to skeletal muscles and is voluntarily
controlled
Autonomic nervous system
Involuntarily regulates the internal environment
Carries signals to cardiac muscle, smooth muscle, and
glands
Peripheral Nervous System
The ANS division have antagonistic effects on target
organs
Sympathetic division: “fight-or-flight” response
Parasympathetic division: promotes a return to self-
maintenance functions and resting and digesting
Parasympathetic division Sympathetic division
Action on target organs: Action on target organs:
Location of
preganglionic neurons:
brainstem and sacral
segments of spinal cord
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Location of
postganglionic neurons:
in ganglia close to or
within target organs
Neurotransmitter
released by
postganglionic neurons:
acetylcholine
Constricts pupil
of eye
Stimulates salivary
gland secretion
Constricts
bronchi in lungs
Slows heart
Stimulates activity
of stomach and
intestines
Stimulates activity
of pancreas
Stimulates
gallbladder
Promotes emptying
of bladder
Promotes erection
of genitalia
Cervical
Thoracic
Lumbar
Synapse
Sympathetic
ganglia
Dilates pupil
of eye
Inhibits salivary
gland secretion
Relaxes bronchi
in lungs
Accelerates heart
Inhibits activity of
stomach and intestines
Inhibits activity
of pancreas
Stimulates glucose
release from liver;
inhibits gallbladder
Stimulates
adrenal medulla
Inhibits emptying
of bladder
Promotes ejaculation and
vaginal contractions Sacral
Location of
preganglionic neurons:
thoracic and lumbar
segments of spinal cord
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Location of
postganglionic neurons:
some in ganglia close to
target organs; others in
a chain of ganglia near
spinal cord
Neurotransmitter
released by
postganglionic neurons:
norepinephrine
Figure 49.8
Types of Neurons
Neurons have a wide variety of shapes that reflect
their input and output interactions
Figure 48.5
Axon
Cell
body
Dendrites
(a) Sensory neuron (b) Interneurons (c) Motor neuron
Types of Neurons
Sensory neurons transmit information from sensory
receptors to the CNS
Detects external stimuli and internal conditions
Interneurons integrate the information in the CNS
This can be in the spinal cord or connect up to the brain
Motor neurons transmit information away from the
CNS
Neurons communicate with effector cells/organs (muscles
and glands)
Stages of Information Processing
Reflex arc – body’s automatic response
to a stimulus
This pathway includes:
Receptor
Sensory neuron
Interneuron
Motor neuron
Effector organ
Reflex Arc
This is a much faster response compared
to the typical stimulus-response
transmission pathways
The reason is that reflex arcs do not involve the
integration of the brain and have fewer neuron
connections compared to other pathways
Reflex arcs also do not require conscious control
and involuntarily occur which leads to some of our
innate responses
Reflex Arc
Figure 49.3
Sensory neurons
from the quadriceps
also communicate
with interneurons
in the spinal cord.
The interneurons
inhibit motor neurons
that supply the
hamstring (flexor)
muscle. This inhibition
prevents the hamstring
from contracting,
which would resist
the action of
the quadriceps.
The sensory neurons communicate with
motor neurons that supply the quadriceps. The
motor neurons convey signals to the quadriceps,
causing it to contract and jerking the lower leg forward.
4
5
6
The reflex is
initiated by tapping
the tendon connected
to the quadriceps
(extensor) muscle.
1
Sensors detect
a sudden stretch in
the quadriceps.
2 Sensory neurons
convey the information
to the spinal cord.
3
Quadriceps
muscle
Hamstring
muscle
Spinal cord
(cross section)
Gray matter
White
matter
Cell body of
sensory neuron
in dorsal
root ganglion
Sensory neuron
Motor neuron
Interneuron
Neuron Structure
Figure 48.4
Dendrites
Cell body
Nucleus
Axon hillock
Axon Signal
direction
Synapse
Myelin sheath
Synaptic
terminals
Presynaptic cell Postsynaptic cell
Neuron Structure
Cell body = contains the organelles
Dendrites = highly branched extensions that receive
signals from other neurons
Axon = cytoplasmic extension that transmits signals to
other cells at synapses
May be covered with Schwann cells which is a fatty cell
wrapped around the axon to form the myelin sheath
Neuron Structure
Nodes of Ranvier = space between the Schwann
cells on the axon
Axon terminals = contains the vesicles of
neurotransmitters (chemical messengers that act as
ligands)
Supporting Cells (Glia)
Essential for the structural integrity of the nervous
system and for the normal functioning of neurons
CNS
Astrocytes – supplies nutrients to neurons in the CNS
Oligodendrocytes – protection
Ependymal cells – lines ventricles and has cilia to move
cerebrospinal fluid
Microglial cells – protection against microorganisms and clean
up cellular debris
PNS
Schwann cells – protection
Myelin sheath
Nodes of
Ranvier
Schwann
cell Schwann
cell
Nucleus of
Schwann cell
Axon
Layers of myelin
Node of Ranvier
0.1 µm
Axon
4 Steps:
- Resting membrane potential
- Depolarization after threshold
- Action Potential
- Repolarization
Nerve Physiology
Nerve Physiology
Membranes of neurons are polarized due to an
electrical potential difference called the resting
membrane potential
The inside of the cell is negative relative to the outside
and is measured using a voltmeter
The resting membrane potential is when a neuron is not
transmitting a signal
Resting membrane potential = - 70mV
Resting Membrane Potential
In all neurons, the resting membrane potential depends
on the ionic gradients that exist across the plasma
membrane
Ion pumps and ion channels maintain the resting potential
of a neuron
CYTOSOL EXTRACELLULAR
FLUID [Na+]
15 mM
[K+]
150 mM
[Cl–]
10 mM
[A–]
100 mM
[Na+]
150 mM
[K+]
5 mM
[Cl–]
120 mM
–
–
–
–
–
+
+
+
+
+
Plasma
membrane Figure 48.6
Resting Membrane Potential
The concentration of Na+ is higher in the extracellular
fluid than in the cytosol while the opposite is true for K+
A neuron that is not transmitting signals contains many
open K+ channels and very few open Na+ channels in its
plasma membrane
The diffusion of K+ and Na+ through these channels
leads to a separation of charges across the membrane,
producing the resting potential
Why is the charge -70 mV?
Figure 48.7
Inner
chamber Outer
chamber Inner
chamber
Outer
chamber –90 mV +62 mV
Artificial
membrane
Potassium
channel
K+ Cl–
150 mM
KCL
150 mM
NaCl 15 mM
NaCl
5 mM
KCL
Cl–
Na+
Sodium
channel
+ –
+ –
+ –
+ –
+ –
+ –
(a) Membrane selectively permeable to K+ (b) Membrane selectively permeable to Na+
Why is the charge -70 mV?
K+ is moved into the cell and Na+ is moved outside
due to the action of the Na/K pump
If K+ is allowed to flow back to equilibrium, the
membrane would be at -90mV
Separately, if Na+ is allowed to flow to equilibrium,
the membrane would be at +62 mV
Why is the charge -70 mV?
Because there are more K+ channels open
compared to Na+ channels AND there are
negative proteins inside the cell, the charge
difference settles to -70mV
Basically, a few positive things are leaking back
into the cell which cancels out some of the -90mV
difference from the K+ flow
Action Potential Steps
Gated ion channels open or close in response to the
binding of a specific ligand or a voltage change
The response is a change in the membrane potential
When ion channels are stimulated, two different
responses can occur: hyperpolarization or
depolarization
Both are called graded potentials because the magnitude
of the change in membrane potential varies with the
strength of the stimulus
Cell Responses
Some stimuli trigger a
hyperpolarization
An increase in the magnitude of
the membrane potential (larger
negative difference from outside
to inside)
Figure 48.9
+50
0
–50
–100
Time (msec) 0 1 2 3 4 5
Threshold
Resting
potential Hyperpolarizations
Me
mb
ran
e p
ote
ntial (m
V)
Stimuli
(a) Graded hyperpolarizations
produced by two stimuli that
increase membrane permeability
to K+. The larger stimulus produces
a larger hyperpolarization.
Cell Responses
Other stimuli trigger a
depolarization
A reduction in the magnitude of
the membrane potential (move
towards a positive difference
from outside to inside)
Figure 48.9
+50
0
–50
–100
Time (msec)
0 1 2 3 4 5
Threshold
Resting
potential Depolarizations
Me
mb
ran
e p
ote
ntial (m
V)
Stimuli
(b) Graded depolarizations produced
by two stimuli that increase
membrane permeability to Na+.
The larger stimulus produces a
larger depolarization.
Cell Responses
A stimulus strong enough to
produce a depolarization that
reaches the threshold will
trigger an action potential
Threshold = membrane voltage
amount needed to cause an
action potential
- 55 mV
Figure 48.9
+50
0
–50
–100
Time (msec)
0 1 2 3 4 5 6
Threshold
Resting
potential
Me
mb
ran
e p
ote
ntial (m
V)
Stronger depolarizing stimulus
Action
potential
(c) Action potential triggered by a
depolarization that reaches the
threshold.
Action Potential Steps
An action potential is a brief all-or-none
depolarization of a neuron’s plasma membrane that
carries information along axons
Both voltage-gated Na+ channels and voltage-gated
K+ channels are involved in the production of an action
potential
Voltage-gated channels rely of electrical signals rather
than ligands
Action Potential Steps
Depolarization
Membrane Na+ channels open which allows Na+ to diffuse
into the cell
This causes the charge on the neuron membrane to change
to positive inside and negative outside
Action Potential
Propagation of the signal is continued depolarization
down the axon
Action Potential Steps
Repolarization
As the action potential subsides K+ channels open, and
K+ flows out of the cell which changes the charge again
on the membrane
Na/K pump restores the ion concentration differences
with the use of ATP
This comes back to the resting membrane potential
A refractory period follows the action potential
during which a second action potential cannot be
initiated
Conduction of Action Potentials
An action potential can travel long distances by
regenerating itself along the axon
The opening of Na+ channels triggers the opening of
even more channels
The speed of an action potential increases with the
diameter of an axon
Conduction of Action Potentials
Action potentials in myelinated axons jump between
the nodes of Ranvier in a process called saltatory
conduction
This allows the signal to travel faster down the axon
Cell body
Schwann cell
Myelin
sheath
Axon
Depolarized region
(node of Ranvier)
+ + + +
+ + +
+ +
+ +
– –
– –
– –
– – –
–
–
–
Figure 48.13
Synapse
In an electrical synapse, electrical current flows
directly from one cell to another via a gap junction
The vast majority of synapses are chemical synapses
In a chemical synapse, a presynaptic neuron releases
chemical neurotransmitters, which are stored in the
synaptic terminal
The neurotransmitters will travel through the space
between the cells called the synaptic cleft to bind to the
post-synaptic neuron
Synapse
Figure 48.14
Postsynaptic
neuron
Synaptic
terminal
of presynaptic
neurons
5 µ
m
Synapse
When an action potential reaches the terminal a
voltage-gated Ca2+ channel opens to allow Ca2+ to
flow into the axon terminal
Ca2+ acts a second messenger and causes the
vesicles holding the neurotransmitters to fuse with
the plasma membrane
The final result is the release of neurotransmitters
into the synaptic cleft
Synapse
Figure 48.15
Presynaptic
cell
Postsynaptic cell
Synaptic vesicles
containing
neurotransmitter Presynaptic
membrane
Postsynaptic
membrane
Voltage-gated
Ca2+ channel
Synaptic cleft
Ligand-gated
ion channels
Na+
K+
Ligand-
gated
ion channel
Postsynaptic
membrane
Neuro-
transmitter
1 Ca2+
2
3
4
5
6
Direct Synaptic Transmission
The process of direct synaptic transmission involves the
binding of neurotransmitters to ligand-gated ion
channels
Neurotransmitter binding causes the ion channels to
open, generating a postsynaptic potential
Postsynaptic potentials fall into two categories:
Excitatory (stimulatory) or Inhibitory
Direct Synaptic Transmission
After its release, the neurotransmitter diffuses out of
the synaptic cleft
May be taken up by the pre-synaptic cell or degraded by
enzymes
Neurotransmitters
Chemical messengers that act on cells to create a
response
The same neurotransmitter can produce different
effects in different types of cells
Types:
Acetylcholine, biogenic amines, various amino acids and
peptides, and certain gases
Neurotransmitters
Acetylcholine is one of the most common
neurotransmitters in both vertebrates and invertebrates
Can be inhibitory or excitatory
Used in muscle contraction
Biogenic amines: include epinephrine, norepinephrine,
dopamine, and serotonin
Are active in the CNS and PNS
Neurotransmitters
Various amino acids and peptides are active in the
brain
Gases such as nitric oxide and carbon monoxide are
local regulators in the PNS
Cerebrum, cerebellum, brainstem, and
diencephalon
Structure of the Brain
Anatomy
Gray matter – no myelin sheath
Located on outside in brain and inside in spinal cord
White matter – has myelin sheath
Located on outside in spinal cord and inside in brain
Gray matter
White
matter
Ventricles
Figure 49.5
Brainstem
The brainstem consists of three parts:
medulla oblongata, pons, and midbrain
The medulla oblongata contains centers that control
heart rate, blood pressure, breathing, swallowing, and
vomiting
The pons controls breathing
The midbrain contains centers for passing ascending
and descending signals
Arousal and Sleep
A diffuse network of neurons called the reticular formation is present in the core of the brainstem
A part of the reticular formation, the reticular activating system (RAS) regulates sleep and arousal
Figure 49.10
Eye
Reticular formation
Input from touch,
pain, and temperature
receptors
Input from ears
Cerebellum
The cerebellum is important for coordination and
balance
Also involved in learning and remembering motor skills
Diencephalon
The embryonic diencephalon develops into three adult brain regions:
epithalamus, thalamus, and hypothalamus
The epithalamus includes the pineal gland (releases melatonin) and the choroid plexus (capillaries that produce cerebrospinal fluid)
The thalamus sends sensory and motor information to the cerebrum
Diencephalon
The hypothalamus regulates homeostasis
Basic survival behaviors such as feeding, fighting, fleeing,
and reproducing
Part of the limbic center
Cerebrum
The cerebrum contains right and left cerebral
hemispheres
Each consist of cerebral cortex overlying white matter and
basal nuclei (regions of gray matter inside brain) – centers
for planning and learning movement sequences
Left cerebral
hemisphere
Corpus
callosum
Right cerebral
hemisphere
Basal
nuclei
Figure 49.13
Cerebrum
A thick band of axons, the corpus callosum provides
communication between the right and left cerebral
cortices
In humans, the largest and most complex part of the
brain is the cerebral cortex, where sensory information
is analyzed, motor commands are issued, and
language is generated
Cerebrum
Each side of the cerebral cortex has four lobes
Frontal, parietal, temporal, and occipital
Frontal lobe
Temporal lobe Occipital lobe
Parietal lobe
Frontal
association
area
Speech
Smell
Hearing
Auditory
association
area Vision
Visual
association
area
Somatosensory
association
area
Reading
Speech
Taste
Figure 48.27
Cerebrum
In the somatosensory cortex and motor cortex neurons
are distributed according to the part of the body that
generates sensory input or receives motor input
Figure 48.28
Tongue
Jaw Lips
Primary
motor cortex Abdominal
organs
Pharynx
Tongue
Genitalia
Primary
somatosensory
cortex
Toes
Parietal lobe Frontal lobe
Emotions
The limbic system is a ring of structures around the
brainstem
Figure 48.30
Hypothalamus Thalamus
Prefrontal cortex
Olfactory
bulb Amygdala Hippocampus
Emotions
This limbic system includes three parts of the cerebral
cortex: amygdala, hippocampus, and olfactory bulb
These structures attach emotional “feelings” to
survival-related functions
Structures of the limbic system form in early
development and provide a foundation for emotional
memory, associating emotions with particular events or
experiences
Memory and Learning
The frontal lobes are a site of short-term memory
Interact with the hippocampus and amygdala to
consolidate long-term memory
Many sensory and motor association areas of the
cerebral cortex are involved in storing and retrieving
words and images
Neural Stem Cells
The adult human brain contains stem cells that can
differentiate into mature neurons
The induction of stem cell differentiation and the
transplantation of cultured stem cells are potential
methods for replacing neurons lost to trauma or
disease
Figure 49.24
